Exercise 1: ChloroMethanes Process


The third example in this section is a process for the manufacture of dichloromethane [DC] and trichloromethane [TC] by successive chlorination of monochloromethane [MC]. Tetrachloromethane, also known as carbontetrachloride [CTC], is produced in significant quantities as a byproduct.

As mentioned previously, this process will be used as a practical example on the use of the Hierarchical Approach to Control System Design. Thus at various stages options will be given as to the next step forward. If the wrong option is taken further advice will be given until the point in question is understood. If you have not looked at the previous three worked examples then it is recommended that you do so. Also it is your own best interest to carry out the following procedures properly and not just skip from page to page!

Process Description

The reactions which describe this process are

The ratios of the three organic products is determined by the ratio of chlorine to fresh monochloromethane feed. This is set as a ratio for a given production programme and is not adjusted by online feedback control.

The HCl is exported as gas to another plant. A certain amount of monochloromethane is acceptable in this stream but is economically undesirable. Dichloro and higher products must be avoided.

The adsorption column C101 uses CTC already present in the process as a solvent to remove the heavier organics from the reactor offgas.

The reactor, absorption column and recycle condenser operate at 5 bara. The rest of the process is at ambient pressure.

As before we will look at the whole process without control before going back to basics and starting from the input/output stage.

Figure 1 - ChloroMethanes Process Without Control

Input-Output Structure

The figure below shows the input-output structure for this process. At this stage it is difficult to give a number of options to choose between but if you are in any doubt as to where this stucture came from then please activate the link below for further information.

Further Explanation

Figure 2 - Input/Output Structure Without Control

Now it is necessary to add control to this structure. As with the first two examples we have to decide whether to control the feed or product rates.

In the process description it is mentioned that the ratios of the three organic products is determined by the ratio of chlorine to fresh monochloromethane feed. Thus it would seem sensible to set this ratio using a controller.

The next issue is concerned with the four product streams. What control is required for them? A short discussion is included on this subject but before looking at the answers please read through the following points.